1600 TP hesitation - cold exhaust pre-heater tubes

[user 18602]

Just got back from a short (25 mile) run and had some hesitation when pulling away on the way home. I immediately suspected that the engine was too cool as it ran fine on the way out and after sunset the air was noticeably cooler.

It's fitted with electronic ignition which is all strobed up correctly. I overhauled the PICT34 carb in the summer with a new gasket set and I think the accelerator pump is giving a sufficient squirt at the right time, so I don't think it's anything like that.

However, I noticed when running it around this summer (I've only just got this on the road and must have done less than 200 miles) that I had hesitation until the motor had been running for ten or fifteen minutes. I noticed that the pre-heater tubes didn't get as hot as I expected (no leaks or anything like that) but didn't worry about it too much.

Can any of you experienced air-cooled people confirm my suspicion that the likely culprit is the pre-heaters? When I got home I popped the engine bay lid and the inlet manifold was stone cold and so were the pre-heater tubes at the manifold end. I think the hesitation was worse after a 55 mph run along about five miles of dual carriage way (during which it ran exceptionally well). My theory is that when running around at low speeds the engine compartment gets warm enough for it not to be a problem but a higher speeds all the heat is drawn from the manifold.

Words of wisdom appreciated

 

[Cali_bay]

Sounds like icing. If your pre-heat tubes are blocked this can cause this problem especially when the weather gets colder.

 

[67panel]

check your exhaust first,have come across some aftermarket ones with no hole for heatriser .saved me having to weld them up for twin carb fitting 8) stock exhaust?then check gaskets after that see if risers blocked with carbon.can be cleaned out but not easy.

 

[user 18602]

Problem solved

The pre-heater tube was completely blocked. Used the bowden cable and drill method which was time consuming but worked really well.

Went out for a spin this afternoon and the pre-heater and manifold now gets hot. I notice that the direction of travel of the exhaust gas through the pre-heater tube is from left to right. The left hand end gets too hot to touch (over boiling point) and the right end is hot but most of the heat given up to the manifold.

Next job: replace the air filter - anyone got one?